Tandem silylformylation-allyl(crotyl)silylation: a new approach to polyketide synthesis

Tandem intramolecular silylformylation-allyl(crotyl)silylation reactions have been developed that allow the highly efficient synthesis of polyketide fragments. The substrates are subjected to Rh(I)-catalyzed silylformylation to afford β-(diallyl)silyl aldehydes which undergo spontaneous uncatalyzed allylsilylation. This unusual spontaneous allylsilylation reaction is driven by strain release Lewis acidity, which arises from the ∼95° O-Si-C bond angle in the oxasilacyclopentane product of the silylformylation reaction. The methodology has been developed both for alkene and alkyne substrates, may be used to establish as many as three stereocenters, and has been shown to be amenable to use in an iterative fashion.     

Application of ESCA to polymer chemistry. VIII. Surface structures of AB block copolymers of polydimethylsiloxane and polystyrene

The surface morphology of a number of films of AB block copolymers of polydimethylsiloxane and polystyrene was examined by ESCA and contact angle measurements. In all cases the immediate surface is shown to consist of an essentially pure polydimethylsiloxane component. By comparing the intensities of elastic peaks corresponding to photoionizations from core levels without energy loss for polydimethylsiloxane and polystyrene with those for the block copolymers and by consideration of shake-up phenomena specific to the polystyrene component, an estimate of the thickness of the polydimethylsiloxane outer layer of the latter may be obtained. This is shown to vary between ～13 and 40 Å, depending on the method of formation of copolymer film.     

A molecular mechanics and semiempirical conformational analysis of the herbicide diuron inhibitor of photosystem II

The potential energy surface (PES) for the herbicide diuron (DCMU), a photosystem II inhibitor, has been extensively investigated using the quantum-mechanical semiempirical molecular orbital methods AM1 and PM3 and molecular mechanics method. A detailed conformational search has been carried out which revealed the occurrence of four genuine minimum energy structures. The relative stability of the conformers and rotational barriers to conformational interconversion were evaluated using distinct theoretical approaches. The results showed that thetrans form of the diuron molecule is more stable than thecis form in all methods, and so it may possibly be the biologically active isomer.     

Cheap control of the time-invariant regulator

The asymptotic solution of the linear quadratic state regulator problem is obtained as the cost of the control tends to zero. Matrix Riccati gains are obtained via singular perturbations theory and are used to asymptotically calculate the optimal control and the corresponding trajectories. Several cases are distinguished and applications are discussed.     

Assay for N tau-methylimidazoleacetic acid, a major metabolite of histamine, in urine and plasma using capillary column gas chromatography-negative ion mass spectrometry

A gas chromatographic-mass spectrometric assay has been developed for the measurement of N tau-methylimidazoleacetic acid in urine and plasma. The method uses the isopropyul ester 3,5-bistrifluoromethylbenzoyl derivative of N tau-methylimidazoleacetic acid and electron capture negative ion chemical ionisation mass spectrometry. The derivative has very good chromatographic properties and a negative ion mass spectrum which contains only a molecular ion at m/z 422. When this ion is specifically monitored, an amount of derivative equivalent to 1 pg of parent compound can be detected. A deuterated analogue of N tau-methylimidazoleacetic acid was synthesised for use as an internal standard and this allowed the development of an assay for N tau-methylimidazoleacetic acid, in urine with a precision of 2.9% and in plasma with a precision of 1.5%.     

Examples Illustrating the Use of Renormalization Techniques for Singularly Perturbed Differential Equations

With nine examples, we seek to illustrate the utility of the Renormalization Group approach as a unification of other asymptotic and perturbation methods.     

A Renormalization Group Method for Nonlinear Oscillators

We present a new approach for finding the asymptotic solution of certain weakly nonlinear oscillator equations. In particular, we develop a simplified version of the renormalization group method of Chen, et al. to obtain higher-order approximations on longer time intervals than are typically provided by averaging and two-timing. The technique, which has much greater potential, is illustrated on three challenging examples from the literature for which better than usual asymptotic solutions are obtained.     

Photoconductive semiconductor switches

Optically activated GaAs switches operated in their high-gain mode are being used or tested for pulsed power applications as diverse as low-impedance, high-current firing sets in munitions; high impedance, low-current Pockels cell or Q-switch drivers for lasers; high-voltage drivers for laser diode arrays; high-voltage, high-current, compact accelerators; and pulsers for ground penetrating radar. This paper will describe the properties of high-gain photoconductive semiconductor switches (PCSS), and how they are used in a variety of pulsed power applications. For firing sets, we have switched up to 7 kA in a very compact package. For driving Q switches, the load is the small (30 pF) capacitance of the Q switch which is charged to 6 kV. We have demonstrated that we can modulate a laser beam with a subnanosecond rise time. Using PCSS, we have demonstrated gain switching a series-connected laser diode array, obtaining an optical output with a peak power of 50 kW and a pulse duration of 100 ps. For accelerators, we are using PCSS to switch a 260 kV, 60 kA Blumlein. A pulser suitable for use in ground-penetrating radar has been demonstrated at 100 kV, 1.3 kA. This paper will describe the specific project requirements and switch parameters in all of these applications, and emphasize the switch research and development that is being pursued to address the important issues